Electrochemical system and method for electropolishing superconductive radio frequency cavities

ABSTRACT

A method and system for electrochemically machining a hollow body of a metal or a metal alloy. An electrode is positioned within a hollow body including a metal or metal alloy, where the hollow body has a variable internal diameter. The hollow body is oriented vertically, with the electrode oriented vertically therein. The hollow body is at least partially filled with an aqueous, acidic electrolyte solution, the electrolyte solution being devoid of hydrofluoric acid and having a viscosity less than 15 cP. An electric current is passed between the hollow body and the electrode, where the electric current includes a plurality of anodic pulses and a plurality of cathodic pulses, and where the cathodic pulses are interposed between at least some of the anodic pulses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/546,072, filed Jul. 11, 2012, the entirety ofwhich is incorporated by reference herein.

GOVERNMENT RIGHTS

This Disclosure was developed under DOE Fermi National AcceleratorLaboratory Purchase Order Number 594128.

FIELD

This application relates to electrochemical machining or polishing ofhollow bodies of metals and metal alloys and, more particularly, tosurface finishing superconductive radio frequency (SCRF) cavities suchas cavities of the type used in supercolliders.

BACKGROUND

The most common fabrication technology for SCRF cavities is to form thinwalled (e.g., about 1 to 3 mm) shell components from high purity niobiumsheets by stamping. These shell components are welded together to formhollow cavities. Horizontal processing of horizontally situated cavitiesas described in U.S. Pat. No. 4,014,765 (Siemens Corporation) wasdeveloped to avoid the adverse effects of gas pockets and bubbleentrainment that lead to nonuniform electropolishing. A schematicillustration of an apparatus for conducting horizontal processing ofSCRF cavities is shown in FIG. 10. The SCRF cavity 100 is orientedhorizontally and mounted on a pair of rotatable end caps 120. One of theend caps includes a circumferential electrically conductive surface 140.A cathode 160 passes through the cavity 100. In the illustratedembodiment, the SCRF cavity includes a single cell which isschematically represented by the large diameter portion in the middle ofthe body shown in the figure. The cathode 160 is electrically connectedto a rectifier 400 by the cathode lead 440. The anode lead 420 of therectifier 400 is connected to the rotating conductive surface 140 whichis electrically connected to the SCRF cavity 100. The cavity 100 ispartially filled with a viscous electrolyte 320. The electrolyte 320 issupplied from a tank 300 via the electrolyte feed tube 340 whichdispenses the electrolyte to the cavity 100. The electrolyte iscontinuously circulated through the cell 100. It leaves the cell througha return tube 360. The volume above the electrolyte 320 in the cell 100contains gas generated during the electropolishing process. This gas ispurged from this space by means of a vent shown schematically at 220.The gas purge 200 is introduced at the end cap 120A on the opposite endof the cell 100. The cavity is rotated on the end blocks 120 as shown bythe directional arrow A in the figure.

One of the vehicles that is often used in electropolishing passivatingmetals like niobium is hydrofluoric acid. As explained herein, theelectrolytes used with these passivating metals tend to be highlyviscous and this can leading to the gas entrainment difficulties thathave required the use of the horizontal processing design discussedabove. Accordingly, there is a need for a method for polishing niobiumand other strongly passivating metals, particularly for use in surfacefinishing SCRF cavities, that does not require the use of highly viscouselectrolytes.

As explained in detail in U.S. Published Application No. 2011/0303553 toInman electrochemical polishing or electrolytic polishing orelectropolishing is a process whereby metal) (M⁰) is selectivity removedfrom a surface by an electrochemical reaction, generally of the form

M⁰→M^(n+)+ne⁻   Eq. 1

As illustrated in FIG. 1, during electropolishing, the currentdistribution is controlled so that the peaks or asperities of thesurface are preferentially removed relative to the recesses or valleysin the subject surface. In the case of primary or geometric currentdistribution as depicted in FIG. 2, the resistive path length from thecathode to the surface asperity (Ωp) is shorter than the distance fromthe cathode to the recess (Ωr). Consequently, the peaks arepreferentially dissolved. The difference in the current distributionbetween the peak and recess is greater as the electrolyte resistanceincreases. Highly resistive electrolytes and low electrolytetemperatures are desirous to increase the differential between thecurrent at the peak and the recess. Decreasing temperature increasesresistivity.

In the case of tertiary or mass transport controlled currentdistribution as depicted in FIG. 3, the diffusion distance from the peakto the bulk solution (Dp) is less than the diffusion distance from therecess to the bulk solution (Dr). Since one skilled in the art wouldunderstand that the diffusion limited current is based on eitherdissolved metal ions diffusing away from the peaks or acceptor ionsdiffusing to the peaks, the diffusion limiting current for metaldissolution at the peaks is higher than the diffusion limiting currentat the recesses. Consequently the peaks are preferentially dissolved.The difference in the diffusion limited current distribution between thepeaks and recesses is higher for viscous solutions. Viscous solutionshave the effect of slowing down the diffusion process. Consequently,highly viscous electrolytes (e.g., about 15 to 30 cP) and lowtemperatures (e.g., 10° C. to 30° C.) leading to higher viscosity areable to increase the differential between the current at the peak andthe recess. Consequently, electropolishing solutions used in the systemsdiscussed above are generally highly resistive (e.g., 10 mS/cm to200mS/cm) and high viscosity (e.g., about 15 cP to 30 cP) solutions, insome cases operating at low temperatures as disclosed by D. Ward“Electropolishing” in Electroplating Handbook ed. L. Durney 4^(th)edition pg. 108, Van Nostrand Reinhold, New York (1984).

Despite the obstacles presented by strongly-bonded passivation layers,various techniques have been developed for electrochemically processingsuch metals as niobium and niobium alloys. In addition to highlyresistive and high viscosity electrolytes, these techniques typicallyrequire high voltages and/or hydrofluoric acid in the electrolytesolution. The electrochemical conditions which drive the reaction shownin Eq. 1 above also drive the following reaction which results in theformation of passivating oxides.

M⁰+xH₂O→MOx+2xH⁺+2xe⁻   Eq. 2

By electropolishing in non-aqueous or minimally aqueous electrolytes,the source of the oxygen that forms these passivating oxides iseliminated. However, maintaining low water content presents anadditional set of control challenges. Using reverse current pulseconditions in accordance with this disclosure provides the means tomanage the formation of this layer of passivating oxides, even in thepresence of substantial water, so that the oxides do not interfere withelectropolishing.

SUMMARY

One manifestation of this disclosure is a method for electropolishingSCRF cavities using relatively low viscosity hydrofluoric acid freeelectrolytes that enable one to process the cavities without gasentrainment. In accordance with another manifestation, the SCRF cavitiescan be processed in a vertical orientation. Another manifestation of thedisclosure is a process that does not require cavity rotation or theneed to purge gases and that may overcome other disadvantages of thehorizontal processing system disclosed in U.S. Pat. No. 4,014,765.Another manifestation is a process for polishing hollow niobium bodiesthat may lead to a cost effective, scalable, high yield process to meetthe demand for SCRF cavities.

According to this disclosure, an electrically mediated approach is usedto eliminate the need for hydrofluoric acid and/or fluoride salts and toreduce the effect of hydrogen in finishing the highly passive metalsurfaces of SCRF cavities. The electrically mediated process isenvironmentally benign and retains the advantages of electrochemicalprocesses in terms of speed and investment. Depending on the flow rateof the electrolyte across the work piece, a waveform is selected topolish the surface. As the surface roughness is reduced andmacroroughness is reduced to microroughness, the waveform may be changedas required. These distinct waveforms can be preprogrammed into therectifier.

In another aspect, the disclosed method may include the steps ofpositioning an hydrofluoric acid free electrolyte solution that mayoptionally be an aqueous electrolyte solution between a workpiece and anelectrode, and passing an electric current between the workpiece and theelectrode, wherein the electric current is comprised of anodic pulsesand cathodic pulses, and wherein the cathodic pulses are interposedbetween at least some of the anodic pulses.

In another aspect the electrolyte solution contains a surfactant such asTriton-X to facilitate the release of oxygen bubbles generated in theelectrolytic process.

Other aspects of the disclosed electrochemical machining system andmethod will become apparent from the following description, theaccompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of electropolishing.

FIG. 2 is a schematic representation of electropolishing with resistiveelectrolytes.

FIG. 3 is a schematic representation of electropolishing with viscouselectrolytes.

FIG. 4 is a schematic illustration of one particular implementation ofthe disclosed electrochemical machining system.

FIGS. 5A and 5B are graphical illustrations of anodic pulse-cathodicpulse waveforms used in connection with the electrochemical machiningsystem of FIG. 4.

FIG. 6 is an example of polarization curves for metal in differentelectrolytes.

FIG. 7 is a two electrode polarization curve for Niobium in differentelectrolytes.

FIG. 8 is a graph showing the effect of Vf on the average surfaceroughness of Nb after electropolishing in 31% by weight H₂SO₄electrolytes at ambient temperature (˜20° C.) using (a) Vf=20 V; (b)Vf=30 V; and (c) Vf=40 V.

FIG. 9 is a graph showing the effect of time on the average surfaceroughness of Nb after electropolishing in 31% by weight H₂SO₄electrolytes at room temperature (˜20° C.) using Vf=30 V, Vr=8 V,frequency=1000 Hz, Df/Dr=10%/90% and different Rao.

FIG. 10 is a schematic of an electropolishing apparatus for horizontalprocessing as disclosed in the prior art.

FIG. 11 is a schematic of an electropolishing apparatus for verticalprocessing a SCRFC in accordance with one embodiment.

FIG. 12 is a schematic of an electropolishing apparatus for verticalprocessing a multi-cavity SCRFC in accordance with another embodiment.

DETAILED DESCRIPTION

U.S. Published Application 2011/0303553 to Inman is incorporated hereinby reference.

As used herein, “electrochemical machining” broadly refers to anyelectrochemical process that involves the removal of material from aworkpiece, regardless of the extent of removal. For example,electrochemical machining includes electrochemical polishing,electrochemical etching, electrochemical through-mask etching,electrochemical shaping and electrochemical deburring, among otherprocesses.

As used herein the term “hydrofluoric acid free” includes electrolytesthat may be formulated to be substantially free of fluoride acids andsalts as well as hydrofluoric acid.

FIGS. 11 and 12 illustrate schematically two embodiments of theinvention wherein the superconducting radio frequency cavity isvertically oriented and electropolished. In FIG. 11, the cavity 100includes a single cell (similar to FIG. 10) whereas in FIG. 12, thecavity 500 includes multiple (nine) cells. A cathode 160 is verticallyoriented in the cells 100 and 500 and is retained by the end blocks 120.The end blocks 120, unlike the end blocks shown in FIG. 10, are notdesigned to rotate nor do they include means for purging gas generatedduring the electropolishing process. This is unnecessary because at theviscosities used, the oxygen gas generated during processing is readilydispersed in the electrolyte such that it does not interfere with thesurface finishing process. The cathode 160 is electrically connected tothe cathode 440 of the rectifier 400. The cell 100/500 is electricallyconnected to the anode 420 of the rectifier 400 by the end plate 120.The electrolyte 300 is introduced to the bottom of the cavity 100/500 bya feed tube 340 from the electrolyte supply tank 300. At the top of thecavity 100/500, electrolyte is expelled from the cavity through the endcap 120 through the electrolyte return tube 360.

The flow of the electrolyte through the cavity is adjusted such that theelectrolyte is refreshed during the electropolishing process.

The power source or rectifier is configured to pass an electric currentbetween the electrode and the workpiece, wherein the electric current iscomprised of anodic pulses and cathodic pulses, and wherein the cathodicpulses are interposed between at least some of the anodic pulses. Theelectrolyte is characterized in that it is hydrofluoric acid and/orfluoride salt free. In one embodiment it is an aqueous solution ofsulfuric acid at concentrations of 70% by weight and less. In certainembodiments, the amount of water in the electrolyte may be greater than5% by weight, or greater than 10%, or greater than 20%, or greater than70%.

The electrolyte solution disclosed herein may be used with an anodicpulse-cathodic pulse waveform to electrochemically machine niobium andalloys thereof, as well as other metals and metal alloys, includingother metals and metal alloys that tend to have strongly bondedpassivation layers. For example, the disclosed electrolyte solution maybe used with an anodic pulse-cathodic pulse waveform toelectrochemically machine niobium and niobium alloys, titanium andtitanium alloys (such as titanium and molybdenum alloys, and titaniumand nickel alloys also know as nitinol), zirconium and zirconium alloys,hafnium and hafnium alloys, tantalum and tantalum alloys, molybdenum andmolybdenum alloys, and tungsten and tungsten alloy. In anotherembodiment the process may be used to polish cobalt chrome alloys of atype used in medical applications such as vascular and other stents.

The disclosed hydrofluoric acid free electrolyte solution in oneembodiment may be an electrolyte having a low viscosity such as aviscosity of about 1 cP to 15cP or about 1 cP to 8 cP, or about 1 cP to4 cP. In a particular embodiment it may be an aqueous solution comprisedof low concentrations of sulfuric acid, for example, concentrations ofabout 1% by weight to 70% by weight, more particularly about 15% byweight to 40% by weight, still more particularly about 20% by weight to40% by weight. In other embodiments of the invention, other acidic,hydrofluoric acid and fluoride acid and salt free electrolytes may beused such as combinations of sulfuric/chromic/phosphoric acids,phosphoric/chromic acids, phosphoric/sulfuric acids, phosphoric acid,phosphoric/sulfuric/chromic acids, phosphoric/sulfuric/hydrochloricacids, sulfuric/glycolic acids, phosphoric/sulfuric acids,sulfuric/chromic acids, sulfuric/citric, and others. Generally, it willbe desirable to select electrolytes having a high conductivity such asgreater than 200 mS/cm, or greater than 400 mS/cm, or greater than 600mS/cm or greater than 800 mS/cm.

In one implementation of the invention, aqueous electrolytes containingsubstantial water as disclosed above may be use. However, anotherimplementation may employ non-aqueous or minimally aqueous electrolytescontaining less than 15%, less than 10% or less than 5% water. Whenwater-containing electrolytes are used, oxygen is generated according tothe equation:

H₂O→2H⁺+O₂+2e⁻   Eq. 3

It has been found that the addition of a surfactant facilitateselectropolishing. One possible reason for this that the surfactantpromotes the formation of small bubbles that do not interfere with thediffusion process by stirring the electrolyte. Conventional surfactantsmay be used for this purpose such as Triton X (polyethylene glycolp-(1,1,3,3-tetramethylbutyl)-phenyl ether), a product of Rohm and Haas,in conventional amounts.

Referring to FIG. 4, one particular implementation of the disclosedelectrochemical machining system, generally designated 200, may includea working chamber 202 defined by a tank 204 and a cover 208, anelectrolyte holding tank 222, a conduit 218, a pump 220, an electrode304, a workpiece 302, a power source 228 and the disclosed electrolytesolution. The working chamber 202 may be in fluid communication with theelectrolyte holding tank 222 by way of a gravity drain 206. A filter 224may be associated with the drain 206 to filter the electrolyte solutionflowing from the working chamber 202 to the electrolyte holding tank222. The working chamber 202 may also be in fluid communication with theelectrolyte holding tank 222 by way of the conduit 218, wherein the pump220 may pump the electrolyte solution from the electrolyte holding tank222 to the working chamber 202, as shown by arrow 234.

Within the working chamber 202, a workpiece holder 210 may be mountednear the bottom of the tank 204 with adequate spacing from the walls andbottom of the tank 204 to allow for drainage of the electrolyte solutioninto the electrolyte holding tank 222. The workpiece 302 may besupported on the workpiece holder 210 and may be connected to a firstelectrical lead 302 of the power source 228.

The workpiece 302 may be any apparatus or thing capable of beingelectrochemically machined. In one embodiment the work piece can be amedical stent. In another embodiment it may be a dental implant. Inanother embodiment it may be RF superconducting cavities that arecomponents of linear particle accelerators. In a first expression, theworkpiece 26 may be any apparatus or thing that is formed from orincludes a metal or a metal alloy. In a second expression, the workpiecemay be any apparatus or thing that is formed from or includes a metal ora metal alloy that forms a strongly-bonded passivation layer. Examplesof metals that form strongly-bonded passivation layers include niobium,titanium and tantalum. In a third expression, the workpiece may be anydevice that is formed from or includes niobium or a niobium alloy. Forexample, the workpiece may be a niobium structure or a portion of aniobium structure that defines RF superconducting cavities in a particleaccelerator. In a fourth expression, the workpiece may be any apparatusor thing that is formed from or includes titanium or a titanium alloy.For example, the workpiece may be a nitinol tube coated with a resistmaterial, wherein the resist material is patterned to define a surgicalstent after electrochemical machining. In another expression theworkpiece may be any device that is formed from or includes cobaltchromium alloy.

An electrode holder 214 may be positioned in the working chamber 202above the workpiece holder 210. The electrode holder 214 may be suppliedwith the electrolyte solution by way of the conduit 218. The electrode304 may be connected to a second electrical lead 232 (opposite polaritythan the first electrical lead 302) of the power source 228 and may besupported by the electrode holder 214 such that the tool holder 214 maymove the electrode 304 in a vertical axis under control of an electrodefeed controller 226.

In one particular aspect, the electrode 304 may include a central boreand the electrode 304 may be connected to the electrode holder 214 suchthat the central bore of the electrode 304 is directed at the workpiece302. During electrochemical processing, the electrolyte solution may bepumped by pump 220 from the electrolyte holding tank 222 to theelectrode holder 214 and, ultimately, to the electrode 304 by way of theconduit 218. The rate of the electrolyte solution flow is hereinreferred to as E_(v). The electrolyte solution may flow through thecentral bore of the electrode 304 and may exit between the electrode 304and the workpiece 302 before returning to the electrolyte holding tank222 by way of the drain 206. The power source 228 may supply electriccurrent to the workpiece 302 and the electrode 304 by way of the firstand second electrical leads 230, 232 in accordance with the disclosedanodic pulse-cathodic pulse waveform.

The spacing between the electrode and workpiece 302 during processingmay be considered an optimizeable parameter and may depend on thecomposition of the electrolyte solution and the type of electrochemicalmachining process being performed, among other factors. For example, thespacing between the electrode and workpiece may range from about 0.5 to20 millimeters or more particularly 0.5 to 10 millimeters for anelectrochemical shaping process, about 5 to about 12 millimeter for anelectrochemical polishing process and about 5 to about 50 millimetersfor an electrochemical deburring process.

As shown in FIG. 5A, an anodic pulse-cathodic pulse waveform, generallydesignated 50, may include a plurality of anodic pulses 52 and aplurality of cathodic pulses 54. One particular implementation is shownin FIG. 5B.

The period T of the waveform is the sum (T=t₁+t₂+t₀+t₁) of the anodicon-time t₁, cathodic on-time t₂, relaxation period t₀ and intermediateperiod t_(i). The inverse (1/T) of the period T of the waveform is thefrequency f of the waveform. The ratio (t₁/T) of the anodic on-time t₁to the period T is the anodic duty cycle D₁ and the ratio (t₂/T) of thecathodic on-time t₂ to the period T is the cathodic duty cycle D₂. Thecurrent density (i.e., current per unit area of the electrode) duringthe anodic on-time t₁ and cathodic on-time t₂ may be referred to as theanodic peak pulse current density and cathodic peak pulse currentdensity, respectively. The anodic charge transfer density Q₁ is theproduct (I₁t₁) of the anodic current density I₁ and the anodic on-timet₁, while the cathodic charge transfer density Q₂ is the product (I₂t₂)of the cathodic current density I₂ and the cathodic on-time t₂.

In a first expression of the anodic pulse-cathodic pulse waveform, theanodic peak current I₁ may range from about 2 to 6 A/cm², the cathodicpeak current I₂ may range from about 8 to 15 A/cm².

In one embodiment, the voltage and on-time of the anodic pulse isadjusted to remove metal from the micropeaks in the surface via Eq. 1above without accumulating a passivating layer thickness via Eq. 2 abovethat cannot be effectively removed during the cathodic duty cycle.Accordingly, the anodic voltage and on-time are adjusted to oxidize themetal on the micropeaks while generating only that amount of passivatingoxides that can be removed by the cathodic pulse. If the passivatingoxide layer cannot be removed, it prevents or terminates polishing. Theappropriate conditions will vary depending on the nature of the metal.In one embodiment a relatively short anodic pulse t₁, typically fromabout 0.01 ms to about 100 ms, preferably from about 0.05 ms to about 10ms, and an anodic duty cycle from about 1 to 60% or from about 5% toabout 60%, preferably from about 1 to 40% or from about 10% to about40%. The cathodic pulse t₂ may have a pulse width from about 0.01 ms toabout 900 ms, preferably from about 0.1 ms or from about 0.5 ms to about90 ms, and a duty cycle from about 40% to about 99% or about 95%,preferably from about 60% to about 99% or about 90%. The relaxationperiod t_(o) may range from about 0 to about 600 s and the intermediateoff period t_(i) may range from about 0 to about 1000 ms. The frequencyf of the waveform 50 may range from about 1 Hertz to about 5000 Hertz,preferably from about 10 Hz to about 2000 Hz and more preferably fromabout 100 Hz to about 2000 Hz or about 100 Hz to 1000 Hz.

At this point, those skilled in the art will appreciate that theparameters of the pulse waveform 50 can be selected to provide uniformmetal removal from the workpiece and thereby provide more accurateconformity of the workpiece. Additionally, the field can be implementedto reduce or anodically consume some of the hydrogen generated at theworkpiece surface and reduce or eliminate the effects of a nonuniformoxide film. The anodic peak current I₁, the anodic on-time t_(i), thecathodic peak current I₂, the anodic on-time t₂, the relaxation periodt₀ and the intermediate off period t₁, may be varied depending on thecomposition of the workpiece 26, the composition of the electrode, thecomposition of the electrolyte solution and the type of electrochemicalmachining process being performed, among other factors to achieve theseobjectives. Furthermore, those skilled in the art will appreciate thatthe voltage and current may be proportional under the circumstances ofthe disclosed system and method and, therefore, the ordinate in FIG. 2could represent either current or voltage, although it is generally moreconvenient in practice to control the voltage. Furthermore, the waveform50 need not be rectangular as illustrated. The anodic pulses and thecathodic pulses may have any voltage-time (or current-time) profile.Rectangular pulses are assumed merely for simplicity. Again, one skilledin the art will recognize that the point in time chosen as the initialpoint of the pulse train is entirely arbitrary. Either the anodic pulseor the cathodic pulse (or any point in the pulse train) could beconsidered as the initial point. The representation with the anodicinitial pulse is introduced for simplicity in discussion. In accordancewith one embodiment, the cathodic voltage is about 4 to 40 volts orabout 4 to 15 volts or about 8 to 35 volts or about 6 to 12 volts, andin one embodiment about 35 volts. This is in contrast to processes inwhich the workpiece is not made from a strongly passivating metal inwhich case a cathodic voltage of 4 volts or less may be satisfactory.The cathodic voltage is used to depassivate the surface, and forstrongly passivating materials the cathodic voltage needs to be greaterthan 4 volts, one skilled in the art can determine the anodic voltagerequired for the desired electrochemical dissolution, i.e. etchingand/or polishing.

Without being limited to any particular theory, it is believed that theintroduction of cathodic pulses between the anodic pulses has the effectof cathodically consuming the nascent oxygen or cathodically reducingthe oxide film, thereby reducing or eliminating the adverse effects dueto the formation of a non-uniform oxide film. Consequently, when thenext anodic pulse is applied, any passive layer that may have formedwill be more easily broken down, and therefore less capable of forminglocal islands of passivity that tend to resist erosion of the underlyingmetal.

EXAMPLES

Niobium foil, 99.9% pure, was purchased from GoodFellow (GoodFellow,Oakdale, Pa.) (FIG. 131) and cut into two different coupon sizes to usefor the electropolishing studies. Final coupon sizes had the followingdimensions: 1) 25.4 mm×25.4 mm×3 mm, and 2) 30 mm×10 mm×3 mm.

As a simple, efficient, and cost-effective screening method, thepolarization curve can be used to select candidate electrolytes. In FIG.6, curve 1 shows the behavior of an active metal, and curve 2 shows thebehavior of a passivated metal. Before the electric field is applied,the metal anode immersed in the electrolyte has a steady-state voltage(E_(ss)). When the power is applied, the electrode voltage will shift inthe positive direction from E_(ss) to E_(ab) (the breakdown voltage).Above E_(ab), the current density rises abruptly due to the dissolutionreactions occurring on the anode (region AB). The dissolution rate ofthe anode metal stops increasing when a limiting current density I_(lim)is reached (BC region), where the metal atoms form metal ions andcompounds with the activating anions and pass into the electrolyte. Thelimiting current density I_(lim) and the ratio of ΔI to ΔE (the slope ofAB on the polarization curve) can be defined as the metal dissolutionrate and current efficiency in the electrolyte, respectively. In regionBC, the current density remains constant (curve 1) or drops to a lowervalue (curve 2) indicating mass transport phenomena that limit the rateof metal ion removal. The products of metal dissolution reach theirsolubility limit and form a loose deposit or passive film on theelectrode surface. If the metal dissolution is conducted in a passiveelectrolyte, the passive film can grow faster than metal ions pass intothe electrolyte, with the result that the current density falls to lowervalues (curve 2). Generally, the limiting current decreases withincreasing electrolyte concentration, due to the decrease in thesolubility of the reaction products. Since the limiting current isstrongly related to diffusion, it can be increased in the pulse/pulsereverse process by increasing the electrolyte flow rate. When the anodevoltage increases to region CD of the polarization curve, the highervoltage can breakdown or remove the passive film and deposits, andincrease the ionization rate of the metal to increase the currentdensity.

The metal brightness and smoothness in different electrolytes can bedirectly observed from polarization tests, providing information as tothe effect of electrolytes on the etching process. In region AB of thepolarization curve metal is eroded. The metal surface roughness is highdue to the different dissolution rates of the various microscopic areason the surface. At high anode voltages (region BC), the metal surfacebecomes smoothed or even polished, as in the case of curve 1. If theanode voltage reaches the CD region, the metal dissolution at highervoltages will lead to a polished surface with macrodefects (such as finelines, striations and pits). The optimal polarization curve should (1)indicate a low breakdown voltage (E_(ab)), (2) have a high ratio of ΔI/ΔE, and (3) provide a smooth and shiny surface.

DC polarization studies were carried out in order to select anelectrolyte that would enable the pulse/pulse reverse process forelectropolishing Nb coupons. The 2-electrode DC polarization studieswere performed on 25.4 mm×25.4 mm×3 mm Nb coupons to study theelectrochemical activity (e.g. total current density) of Nb in differentelectrolyte type and concentrations. A platinum coated Nb mesh was usedas the cathode. All polarization curve experiments were performed atroom temperature (˜20° C.). A TecNu power supply was used for this study(Model SPR-300/100/48-3). The cell voltage was raised by increments offive volts per minute. Total current densities were read from theoscilloscope trace recorded on a FLUKE 196C Scopemeter color system.

FIG. 7 summarizes the electrochemical activity of Nb substrates indifferent electrolytes, 200 and 300 g/L sodium chloride (NaCl), 31% byweight sulfuric acid (H₂SO₄), 200 g/L sodium bromide (NaBr), 50 g/Lsodium fluoride (NaF) and 21% by weight phosphoric acid (H₃PO₄). In allcases breakdown of the Nb was not observed; any current measured isassumed to be associated with water oxidation (2H₂O→O₂+2H⁺+2e⁻) and Nbanodization. The highest and lowest total current density observed forvoltages up to 70 V was in the 31% by weight H₂SO₄ and 50 g/L NaFelectrolytes, respectively.

This data demonstrated the tenacity of the Nb oxide film; DCpolarization studies were unable to shed any light on the conditionsthat would be required to break down the oxide film without the use ofhydrofluoric acid. Electrochemical cells with variable flow as shown inFIG. 4 were used to test the efficacy of pulse/pulse reverse waveformsin electropolishing Nb. As described in the prior art, variable flowchannel cells are used to successfully perform metal removal of passivemetal and alloys. ^(1, 2) An advantage of electrolyte flow is theremoval of undesired byproducts from the surface of the substrate beingelectropolished, such as Nb ions, heat and bubbles (resulting mainly dueto oxygen and hydrogen generation from water electrolysis). ¹ A.Lozano-Morales, A. Bonifas, M. Inman, P. Miller and E. J. Taylor, J.Appl. Surf. Finish., 2 (3), 192-197 (2007).² J. J. Sun, E. J. Taylor, RSrinivasan, J. Materials Processing Technology, 108 356-368 (2001).

As reported in the prior art, a 300 g/L NaCl electrolyte has beensuccessfully used for pulse/pulse reverse electropolishing differentpassive materials such as nickel based alloys and stainless steel, andtherefore it was used to initially study the electrochemical activity ofNb. An initial design of experiments set was developed using statisticalsoftware called MINITAB®. Frequency, duty cycle and reverse (cathodic)voltage were varied with three levels for each variable. Three differentfrequencies were varied at 10, 100 and 1000 Hz at three differentforward (anodic) duty cycles (D_(f)=10, 50 and 90%), and three differentreverse voltages (V_(r)=2, 4 and 8 V). A Nb coupon of the same dimensionas the anode was used as the cathode. The electrolyte velocity was keptconstant at 12 m/s and a forward (anodic) voltage (V_(f)) of 48 V wasused in all the experiments at ambient temperature (˜20° C.). Thetemperature of the electrolyte was not controlled. The total run time ineach case was 10 minutes.

While uniform etching was not achieved in 300 g/1 NaCl, there wasevidence of Nb breakdown at the coupon edges, where the electrolyte flowentered and exited the cell. Oxides were also formed on the surface,indicating electrochemical activity. These oxides were not tenacious,able to be removed using a scotch-brite pad, soap and water. However,high electrolyte flows and very close electrode gaps are not likely tobe realistically accommodated.

In initial experiments in 31% by weight H₂SO₄, the constant parameterswere electrolyte velocity (E_(v))=0.4 m/s, V_(f)=20 V, V_(r)=8 V, runtime=10 minutes, anode to cathode distance=5 mm, and ambient temperature(˜20° C.). Since the low flow channel cell did not have temperaturecontrol built in, the electrolyte temperature rose from an initial valueof 20° C. to around 27° C. by the end of every experiment. Inconventional electropolishing, temperatures above 40° C. must be avoidedin order to prevent etching pits on the Nb substrate.³ A design ofexperiments set was performed using MINITAB®. Two different frequencieswere used (10 and 1000 Hz) at two different duty cycles (D_(f)=10 and90%), (see Table 1), for a total number of 3 experiments. ³ L. Lilje, E.Kako, D. Kostin, A. Matheisen, W.-D Moller, D. Proch, D. Reschke, K.Saito, P. Schmuser, S. Simrock, T. Suzuki, and K. Twarowski, NuclearInstruments and Methods in Physics Research A 524 1-12 (2004).

TABLE 1 Design of experiments using reverse pulse waveforms toelectropolish Nb in a 31% by weight H₂SO₄ electrolyte for 10 minutesusing a V_(f) = 20 V and anode-cathode distance of 5 mm. Forward(Anodic) Reverse Voltage Run Order Frequency (Hz) Duty Cycle (%) (V_(r))1 10 90 8 2 1000 10 8 3 1000 90 8

Run 1 showed no evidence of etching at all. The different colorsobserved represent Nb oxide layers formed on the substrate. Run 2 showedsome degree of etching, which suggested that Nb substrates could beuniformly electrochemically etched in an electrolyte free ofhydrofluoric acid. Run 3 also showed some degree of etching, but muchlower compared to Run 2.

Based on these preliminary results, the pulse/pulse reverse waveformused for Run 2 was further explored. Specifically, the same waveformparameters as Run 2 were used, but the coupon was electropolished for 37minutes instead of 10 minutes. 100 μm of Nb was successfully removeduniformly from the coupon at an average removal rate of 2.7 μm/min in anarea approximately 161 mm².

The effect of raising V_(f) from 20 to 30 to 40 V on Nb electropolishingperformance was studied. In all cases, the other pulse/pulse reverseprocess parameters were kept constant: V_(r)=8 V, frequency=1000 Hz,D_(f)/D_(r)=10%/90%. FIG. 8 summarizes the effect of anodic peak voltageon Nb surface finish. The roughest surface finish was obtained at 20 V.When V_(f) was increased to 30 V, the surface finish of Nb dropped from1.38 μm to 0.29 μm. At 40 V the Nb surface finish started gettingrougher again and surface discoloration was observed.

The effect of Nb initial surface roughness, Ra_(o) on final surfaceroughness, Ra_(f) was also studied by performing electropolishing timestudies at different Ra_(o). FIG. 9 compares the effect of time on theaverage surface roughness of a Nb coupon after electropolishing in a 31%by weight H₂SO₄ electrolyte for Ra_(o)=0.56 μm (Run #4) and Ra_(o)=1.53μm (Run #5). For the higher initial surface roughness, there was asignificant decrease in Ra after 10 minutes, from 1.53 μm to ˜0.85 μm.Thereafter, Ra decreased further by increasing electropolishing time upto 60 minutes down to an Ra_(f) of 0.33 μm. Further decreases in surfacefinish did not occur after 50 minutes.

Accordingly, electrochemical machining systems and methods employing thedisclosed electrolyte solution with the disclosed anodic pulse-cathodicpulse waveform may be used to process niobium and niobium alloys, aswell as other metals and metal alloys, without the need for fluorideacids or salts, such hydrofluoric acid.

Although various aspects of the disclosed electrochemical system andmethod for machining niobium and other metals have been shown anddescribed, modifications may occur to those skilled in the art uponreading the specification. The present application includes suchmodifications and is limited only by the scope of the claims.

What is claimed is: 1-20. (canceled)
 21. A method for electrochemicallymachining a hollow body of a metal or metal alloy, the methodcomprising: positioning an electrode within a hollow body comprising ametal or metal alloy, wherein the hollow body has a variable internaldiameter; orienting the hollow body vertically, with the electrodeoriented vertically therein; at least partially filling the hollow bodywith a hydrofluoric acid free electrolyte solution having a viscosityless than 15 cP; and passing an electric current between the hollow bodyand the electrode, wherein the electric current is comprised of aplurality of anodic pulses and a plurality of cathodic pulses, andwherein the cathodic pulses are interposed between at least some of theanodic pulses.
 22. The method of claim 21, wherein the electrolytesolution has a viscosity less than about 4 cP.
 23. The method of claim22, wherein the electrolyte solution has a conductivity greater thanabout 200 mS/cm.
 24. The method of claim 23, wherein the voltage and ontime of the anodic pulses are adjusted to polish the hollow body whilelimiting the formation of passivating metal oxide to a thickness thatcan be removed effectively by the cathodic pulse.
 25. The method ofclaim 24, wherein the cathodic pulse voltage is greater than 4 V. 26.The method of claim 21, wherein the metal or metal alloy forms astrongly-bonded passivation layer during the anodic pulses of thepassing an electric current step.
 27. The method of claim 26, whereinthe metal or metal alloy of the hollow body is selected from the groupconsisting of niobium and niobium alloys, titanium and titanium alloys,zirconium and zirconium alloys, hafnium and hafnium alloys, tantalum andtantalum alloys, molybdenum and molybdenum alloys, tungsten and tungstenalloys, and chromium cobalt alloys.
 28. The method of claim 27, whereinsaid hollow body comprises niobium or niobium alloy.
 29. The method ofclaim 21, wherein the electrolyte contains at least about 10% water. 30.The method of claim 21, wherein the hollow body does not rotate duringthe passing an electric current step.
 31. A method for electrochemicallymachining a hollow body of a metal or metal alloy, the methodcomprising: positioning an electrode within a hollow body comprising ametal or metal alloy, wherein the hollow body has a variable internaldiameter; orienting the hollow body vertically, with the electrodeoriented vertically therein; at least partially filling the hollow bodywith an aqueous electrolyte solution having a viscosity less than 15 cP;and passing an electric current between the hollow body and theelectrode, wherein the electric current is comprised of a plurality ofanodic pulses and a plurality of cathodic pulses, and wherein thecathodic pulses are interposed between at least some of the anodicpulses.
 32. The method of claim 31, wherein the electrolyte solution hasa viscosity less than about 4 cP.
 33. The method of claim 32, whereinthe electrolyte solution has a conductivity greater than about 200mS/cm.
 34. The method of claim 33, wherein the voltage and on time ofthe anodic pulses are adjusted to polish the hollow body while limitingthe formation of passivating metal oxide to a thickness that can beremoved effectively by the cathodic pulse.
 35. The method of claim 34,wherein the cathodic pulse voltage is greater than 4 V.
 36. The methodof claim 31, wherein the metal or metal alloy forms a strongly-bondedpassivation layer during the anodic pulses of the passing an electriccurrent step.
 37. The method of claim 36, wherein the metal or metalalloy of the hollow body is selected from the group consisting ofniobium and niobium alloys, titanium and titanium alloys, zirconium andzirconium alloys, hafnium and hafnium alloys, tantalum and tantalumalloys, molybdenum and molybdenum alloys, tungsten and tungsten alloys,and chromium cobalt alloys.
 38. The method of claim 37, wherein saidhollow body comprises niobium or niobium alloy.
 39. The method of claim31, wherein the electrolyte contains at least about 10% water.
 40. Themethod of claim 31, wherein the hollow body does not rotate during thepassing an electric current step.